Patent GB-1392986-A (GB)
Title: Compressing and/or expanding circuit
IPC: H04B1/64
Filed: 1972-04-24
Abstract: 1392986 Compandors NIPPON VICTOR KK 24 April 1972 [24 April 1971] 18875/72 Heading H4R A compressor (or expandor), includes a control element the resistance of which varies in response to a control signal applied thereto and is characterized in that there is provided a signal level adjusting circuit to which an output from said compressor (or an input to said expandor), is applied as an input for producing a control signal for application to said control element, the signal level adjusting circuit comprising at least one amplifier to which a circuit, including non-linear and linear circuit elements, connected in series, is connected as its load and imparting a small attenuation to said input for producing said control signal when the level of said input is low and imparting a larger attenuation to said input when the level of said input is high. In the case of a compressor the control element is connected so as to shunt a variable amount of the input to the compressor to earth and in the case of an expandor the control element is arranged in a negative feedback loop of an amplifier contained in the expandor. The control element is preferably a transistor and it is stated that the inclusion of the signal level adjusting circuit results in a decrease in non-linearity in the compression or expansion range when employing transistor control elements which are inherently non- linear. Fig. 3, shows one embodiment of the invention in which a signal applied to terminal 10 is first compressed before being supplied to a transmission system or to a recording medium such as magnetic tape. The output of the transmission system is applied to an expandor and an output is derived at terminal 40. The compressor, includes a control element in the form of transistor 16a which acts as a variable impedance in accordance with a control voltage applied to its base. This control voltage is derived from the compressor output via a signal level adjusting circuit 20a and a control circuit 21a. The signal level adjusting circuit comprises two amplification stages. The first stage comprising a, transistor 24a, a linear resistance 26a, and a non-linear resistance formed by back-to-back diodes 27a, 28a imparts a small attenuation to signals in a low level range. The second stage comprising transistor 29a, resistor 31a, and diodes 32a, 33a imparts a large attenuation to signals in a high level range. The control circuit 21a then provides a D.C. output for biasing the control element transistor 16a. The expandor circuit contains an identical control circuit 21b and signal level adjusting circuit 20b. In this case the control element transistor 16b is connected in a negative feedback loop of an amplifier 37. It is stated that the range of signal levels and the linearity of compression and expansion can be increased by increasing the number of amplifier stages in the signal level adjusting circuit, each stage providing attenuation in a particular signal level range (Figs. 5a and 5b, not shown). In Fig. 6 (not shown), a compandor circuit is shown which has a changeover switch for switching from expansion to compression. This circuit is based on the circuits shown in Fig. 3, and utilizes the fact that the compressor and expandor shown in Fig. 3, contain similar circuitry. Such a compandor may be used in a tape recorder and the changeover switch would then be associated with the record/playback controls. An F.E.T. is used as the control element in this circuit.